Collaborative Research: Marginal instability and deep cycle turbulence in the equatorial oceans

合作研究：赤道海洋的边缘不稳定和深循环湍流

• 批准号: 1355856
• 负责人: Sutanu Sarkar
• 金额: $ 30.44万
• 依托单位: University of California-San Diego
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2014
• 资助国家: 美国
• 起止时间: 2014-03-01 至 2018-02-28
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1355856&HistoricalAwards=false
• 关键词: Collaborative; Research; Marginal; instability; deep

Overview: Thirty years after its discovery, the deep cycle of equatorial turbulence remains unexplained. Climate forecast models are unable to represent it accurately, leading to errors in ocean heat uptake. This project brings together two research groups who have made recent progress on the problem to pursue a unified understanding. Combining existing observations and new large-eddy simulations (LES), they will explore deep cycle physics over the range of seasonal and ENSO variability and in all three equatorial oceans. The previously-unexploited deep cycle property of marginal instability will be used together with numerical simulations to (1) document the history of the deep cycle over the past 25 years, (2) describe the mechanics of the deep cycle, and (3) develop improved parameterizations for use in climate models.Intellectual merit: The broad variability of mixing in the upper equatorial Pacific is exemplified by the turbulent diapycnal heat flux sampled at the same location and in the same season, but coinciding with different phases of the El Niño/ Southern Oscillation (ENSO) cycle. Early in the 1991 El Niño, currents were slow and the heat flux was relatively weak. In the 2008 cruise, which coincided with La Niña, currents were much more energetic. The resulting heat flux was stronger by an order of magnitude, and heat was transported to 100 m depth. Despite the extreme difference in magnitude, these mixing regimes exhibited a striking commonality: strong turbulence coincided with a distinct layer in which the gradient Richardson number (Ri) remained within a factor of two of the value of a quarter. Persistent clustering of Ri near 1/4 over a range of depths signifies the state of marginal instability. This near-critical state is maintained in the mean by a balance between large-scale forcing (which reduces Ri) and turbulence (which increases it). Deep cycle mixing is crucial to the climate via its effects on both the zonal current system and the sea surface temperature. In this observational data analyses and large eddy simulations, the investigators will explore the physics of the deep cycle with particular attention to the property of marginal instability. Using marginal instability as a proxy, they will document the history and longitudinal dependence of the deep cycle from existing data. This will lead to both a deeper understanding of the phenomenon and an improved ability to parameterize its effects in large-scale models.Broader Impacts: The causes of long-term Sea Surface Temperature variations will be examined and the results will contribute to ENSO and interdecadal climate prediction. The project will also foster a new collaboration, support a junior researcher, and complement an existing NSF observational project. Two presentations on equatorial oceanography will be developed and presented at high schools and community colleges.

概述：在发现30年后，赤道湍流的深循环仍然无法解释。气候预测模型无法准确地表示它，导致海洋热量吸收的错误。该项目汇集了两个在该问题上取得最新进展的研究小组，以寻求统一的理解。他们将结合现有的观测和新的大涡模拟，探索季节和厄尔尼诺/南方涛动变化范围内以及所有三个赤道海洋的深循环物理学。以前未被利用的边缘不稳定性的深循环特性将与数值模拟一起用于（1）记录过去25年深循环的历史，（2）描述深循环的机制，（3）开发用于气候模式的改进参数化。赤道上太平洋混合的广泛变化可以通过在同一地点和同一季节采样的湍流横尺度热通量来说明，但与厄尔尼诺/南方涛动（ENSO）周期的不同阶段相吻合。在1991年厄尔尼诺现象初期，洋流缓慢，热通量相对较弱。在2008年的巡航中，恰逢拉尼娜现象，洋流的能量要大得多。由此产生的热通量强了一个数量级，热量被输送到100米深。尽管在幅度上存在极大的差异，但这些混合制度表现出惊人的共性：强湍流与梯度理查森数（Ri）保持在四分之一值的两倍以内的独特层相吻合。在一定深度范围内，Ri在1/4附近的持续聚集表示边缘不稳定的状态。这种近临界状态是通过大尺度强迫（降低Ri）和湍流（增加Ri）之间的平衡来维持的。深循环混合通过其对纬向流系统和海表温度的影响对气候至关重要。在这次观测数据分析和大涡模拟中，研究人员将探索深循环的物理学，特别注意边缘不稳定性的性质。利用边际不稳定性作为替代，他们将从现有数据中记录深周期的历史和纵向依赖性。更广泛的影响：将研究海表温度长期变化的原因，研究结果将有助于厄尔尼诺/南方涛动和十年期气候预测。该项目还将促进新的合作，支持初级研究人员，并补充现有的NSF观测项目。将编写两份关于赤道海洋学的报告，并在中学和社区学院进行。